There are known various types of forklift trucks. A forklift truck is used to lift and transport materials with a pair of elevationally movable forks for loading and unloading tasks. Typically, the forklift truck has a mast assembly mounted upright on the front of the truck frame and supporting loading forks for elevational movement (See, for example, Patent Publication No. 1 cited below).
The known mast assembly disclosed in Patent Publication No. 1 includes an outer mast having a pair of right and left uprights mounted upright on the front of the truck frame, an inner mast having a pair of right and left uprights each slidably fitted in the corresponding one of the pair of uprights of the outer mast and elevationally movable by means of a lift cylinder, and a carriage assembly slidably fitted in the pair of uprights of the inner mast and elevationally movable by means of the lift cylinder. The carriage assembly is provided with a pair of object carrying forks mounted thereon.
This type of a mast assembly includes guide units disposed between each upright of the outer mast and the corresponding upright of the inner mast, as well as between each upright of the inner mast and the carriage assembly. By virtue of these guide units, the sliding movement of the uprights of the inner mast relative to the uprights of the outer mast, as well as the sliding movement of the carriage assembly relative to the uprights of the inner mast can be caused smoothly by the lift cylinder. Conventionally, various roller guide units are used for such guide units. In other words, various roller guide units have been used in order to guide the inner mast and the carriage assembly for elevational movement. There have been many prior art documents which show such roller guide units in detail (See, for example, Patent Publication No. 2 cited below).
Patent Publication No. 1: Published Japanese Patent Application No. 2005-194007.
Patent Publication No. 2: Published Japanese Patent Application No. Hei-03-232698 (No. 1991-232698).